Conventional defence systems use one rotating radar unit to find a target and a second to subsequently track it for risk assessment.

BBC: News | In Depth | Military Fact Files

Today, thanks to radar and interplanetary spacecraft, the astronomical unit is known with a precision of one part in 50 billion.

ECONOMIST: Transits of Venus

On Day 44 of the trek, both a radar device meant to measure the ice thickness and a satellite communications unit to relay the data are still not working - despite being brought back to the UK for repairs and then delivered to the team last week.

BBC: Arctic team gives up on ice radar